The invention relates to a device for the variable control of the valves of internal combustion engines, more particularly for the throttle-free load control of 4-stroke engines via the intake stroke functions of one or more intake valves per cylinder. Two camshafts rotate to opposite hands and act via a transmission member, more particularly a rocking lever on the or each valve spring-loaded in the closure direction, one camshaft determining the opening function and the second camshaft the closing function, so that the stroke and/or duration of opening of the or each valve can be changed in relation to one another over wide ranges by a relative rotation of the two camshafts.
Such a valve control system is known from Offenlegungsschrift DE-OS 35 31 000. In that valve drive the required variability of a valve control system, principally to avoid throttle losses, is effected by the feature that the opening and closure operation is performed by two different control cams running at a controllable phase angle to the crankshaft. A control lever of any desired construction is so actuated by the two camshafts that the valve spring-loaded in the closure direction is opened only when both control cams are extended. In this way variable valve control times can be adjusted by a suitable phase position of the camshafts. A similar valve control system for intake valves of reciprocating piston internal combustion engines is disclosed in DE-OS 35 19 319 to which U.S. Pat. No. 4,714,057 corresponds. In that case, in addition to a rotating stroke camshaft, a control camshaft rotating at the same speed engages at a displaceable bearing place of the pivotable valve lever. In principle variable valve control systems can be obtained in this way, wherein the course of the valve stroke can be so altered as to reduce the gas exchange losses caused in 4-stroke engines by throttling.
In the system disclosed in DE-OS 35 31 000 the relative rotation of the two camshafts takes place via accelerator-controlled camshaft driving wheels, which can be displaced on corresponding steep threads. Only small angles of rotation with relatively long adjustment times are also permitted by the camshaft phase adjusters, known from other Patent Specifications and Offenlegungsschriften (e.g., DE-OS 29 09 803), some of which are already in serial production, which operate on the principle of the axial displacement of a piston on a helical groove. Moreover, the prior art systems occupy a large constructional space, more particularly in the direction of the engine longitudinal axis.
To achieve throttle-free load control over the whole operating range of present-day motor vehicle 4-stroke engines, relative angles of rotation between the two camshafts of an order of magnitude of 150.degree. to 220.degree. crankshaft are required, if the intention is also to use the potential of optimum valve control times for maximum filling under full load over the whole speed range. Moreover, due to the demands of dynamic vehicle operation, the adjusting process must take place within very short periods of time (fractions of seconds). The adjuster itself should be of compact construction, to meet present-day spatial conditions in the engine chamber.
DE-PS 470 032 discloses a valve control system for internal combustion engines which is mainly characterized in that to control the valve two non-circular control discs are provided whose axes of rotation always maintain their position in relation to the axis of rotation of a transmission lever. The valve-actuating transmission lever takes the form of a two-part rocking lever which has a fixed pivot and which, when the two plate cams rotate in relation to one another, can correspondingly change within narrow limits only the duration of opening or closing of the valves, but not the valve stroke. It is a so-called OR circuit wherein the valve stroke is always determined by the control disc having the maximum operative stroke circle. To avoid jumpy functioning with consequent impermissibly high accelerations in valve operation when the two control discs rotate in relation to one another, a transition from one control disc to the other can in fact only be made with a constant operative stroke, essentially with the maximum stroke. As a result, the usable adjustment range of that system is heavily limited and unsuitable for throttle-free load control. The epicyclic gear for driving a control disc as disclosed in this citation is at the same time used to rotate the two control discs in relation to one another. The epicyclic gear consists of four toothed wheels, of which two toothed wheels are disposed on the parallel shafts of the two control discs and are driven via two further serially connected intermediate wheels. The two intermediate wheels are borne by a movable arrangement of links which gives them an epicyclic motion. The arrangement of links consists of three individual links, of which two links each connect a toothed wheel disposed on the shafts of the control discs to an intermediate wheel, while the third link interconnects the two first-mentioned links. The two links are however not connected to the pivots of the two intermediate wheels, but at some distance therefrom. However, this arrangement of the third link permits an adjustment of the epicyclic gear only when the links bearing the intermediate wheels, the third link and a plane lying in the axes of rotation of the two control discs are disposed parallel with one another. The arrangement of the links of the epicyclic gear must in practice have the shape of a parallelogram, since only in that case do the distances of the two opposite links remain identical for every position of the arrangement of links, something which for this kind of arrangement of links is the basic precondition for the satisfactory functioning of the meshing gear wheels. As a result, of course, the diameters of the four engaging gear wheels are directly dependent on one another, the transmission ratios between the toothed wheels disposed on the shafts of the control discs and the intermediate wheels being predetermined within close limits. More particularly, the diameters of the toothed wheels cannot be freely selected to influence the sensitivity of the angle of rotation of the control shaft to be rotated.